The present invention relates to a method and device for inspecting a printed image.
According to a prior art printed image inspection device, a line sensor camera 2 scans a printed image 1 in the widthwise direction as shown in FIG. 1 so that image information is derived. The angle of rotation of a transfer cylinder 3 is detected by a rotary encoder 4 attached thereto so that in response to the output from the encoder 4, the scanning by the camera 2 is synchronized so that a sample image 5 (See FIG. 2) is divided into a plurality of picture elements in the form of a matrix.
A reference image 6 (See FIG. 2) is also divided into a plurality of picture elements in the form of a matrix. The density of each picture element of the sample image 5 is compared with the density of each picture element of the reference image 6, whereby whether the sample image 5 has defective picture elements or not is detected.
However, if the sample image 5 is deviated widthwise with respect to the reference image 6 as shown in FIG. 3, the gradation 7 of three picture elements (marked by 0) of the sample image 5 are different from the gradation 8 of the corresponding picture elements of the sample image 5. As a result, even though the sample image 5 has no defective picture element, it is detected as unacceptable or as having defective picture elements.
One of the methods to overcome this problem is to provide a large degree of gradation difference tolerance; that is, the tolerance in gradation between the reference and sample images 6 and 5, but this method is not satisfactory in practice because the defects of the sample image cannot be detected with a satisfactorily high degree of accuracy.
Another method to overcome this problem is to correct the misalignment or misregistration between the reference and sample images and various methods of this type have been proposed. For instance, one method is disclosed in the Japanese Patent Application No. 146,355/1981 filed by the same applicant. According to this method, the widthwise position of a sample printed image is detected and stored in a memory. The widthwise position of the sample printed image is then compared with the widthwise position of a reference image stored in a reference-position memory and if the difference between their widthwise positions is in excess of a predetermined distance, sample image data is stored in the reference/position memory.
According to this method, however, means for detecting the deviation in widthwise direction of a sample printed image with respect to a reference image as well as means for correcting the widthwise deviation of the sample printed image are needed. As a result, hardware becomes complex in construction and expensive in cost.
Another method is disclosed in the Japanese Patent Publication No. 45,948/1980. According to this method, a picture element of a sample printed image is compared with a few picture elements adjacent to the corresponding picture element of a reference image. However, this method is still unsatisfactory in practice because a desired degree of inspection accuracy cannot be attained.